Few carbon-cutting practices have raised as much controversy, even among clean energy advocates, as the practice of carbon capture and storage. The basic concept is simple enough: catch carbon dioxide from factories and other industrial facilities before it goes into the atmosphere and then either store it indefinitely underground or inject it into oil reservoirs to help pump out more oil.
But while many experts have touted the process as an essential factor in the mitigation of climate change, others have argued that it’s too risky, too costly and distracts policymakers from the expansion of renewable energy sources, like wind and solar.
While the debate isn’t likely to be resolved any time soon, a new study — just published Thursday in the journal Nature Communications — has addressed at least one of the concerns associated with carbon storage: its safety. In the past, critics have suggested that carbon dioxide stored underground may be able to corrode the rock layers above it and eventually escape, a possibility that’s been supported by some modeling and laboratory studies. This would be bad for the climate, of course, but some environmental and public health advocates have also worried that escaped carbon dioxide in large volumes could damage the water or air quality of nearby communities.
But the new study suggests that such concerns may be overblown. The researchers examined a natural carbon dioxide reservoir near Green River, Utah, and found that the carbon dioxide has been trapped underground there for about 100,000 years without dangerously corroding the rocks that are trapping it in place. (For perspective, climate experts have suggested that carbon dioxide must be kept stored underground for at least 10,000 years to keep it from adding to the current global warming.
These observations suggest that storing carbon underground may (at least at some sites) be much safer than previous model and laboratory experiments have suggested.
To effectively store carbon underground, it’s necessary to pump it down at least a kilometer (that’s about six-tenths of a mile) into the earth, said Mike Bickle, director of the Cambridge Center for Carbon Capture and Storage and senior author of the new study.
“You need a porous rock that’s got enough space to pump the carbon dioxide in,” he added. These types of porous rock are typically already filled with briny water, which the pumped carbon dioxide will dissolve into. Because the carbon dioxide is still less dense than the briny water it replaces, though, it has a tendency to push its way upward. So the key to keeping it trapped underground is to make sure there’s an impermeable layer of rock above it as a “cap rock.” This is the rock layer that critics worry might wear down over time.
The researchers acknowledge that conditions in the Utah reservoir are not identical to those that would be ideal for a man-made carbon storage situation — the natural reservoir is shallower and pressure is therefore lower than the sites that would probably be selected for future storage initiatives. But the researchers argue that the relevant reactions taking place between the carbon dioxide and the rocks and minerals underground are little influenced by pressure, anyway.
The study was also funded in part by Shell, which has several of its own carbon capture and storage projects around the world, including one at the Gorgon gas fields in Australia and others in Canada and Norway. The paper also discloses the fact that three of the 13 authors (Bickle not included) are employed by Shell Global Solutions International. None of the other authors has stated any financial competing interests, and no other conflicts of interest have been disclosed.
To conduct their study, the researchers drilled down hundreds of meters and collected samples of both the cap rocks and the fluids in the reservoir below. They analyzed both the rocks and the fluids to find out what kinds of substances they were composed of, and they also examined the rocks to see how much corrosion had taken place. The researchers also used models to recreate the chemical reactions taking place between the carbon dioxide saturated brine and the rock above it.
The researchers’ simulations confirmed that the reservoir has existed for about 100,000 years. But despite being so old, the scientists found that the cap rock had only been corroded through a layer about seven centimeters (not quite three inches) thick, which is significantly less than the several meters of corrosion previous modeling and laboratory studies have suggested might be likely over such a time frame.
The researchers’ analyses and simulations helped them understand why this was the case. It turns out that certain chemical reactions happening at the cap rock layer have acted as a kind of buffer against the corrosive effects of the carbon-saturated brine.
Of course, different reservoirs may have different conditions and contain different types of minerals, so the researchers caution that the safety of storage sites should always be evaluated on an individual basis. But they also point out that when samples are taken in the field and accompanied by simulations informed with all the relevant geological and chemical complexities, an accurate picture of the reactions taking place at any given site — and their resulting effects on the cap rocks — can be drawn. And the study suggests that the results, using such methods, may end up being much more optimistic than previous studies have suggested.
“The [previous] modeling studies don’t necessarily take into account all the minerals in the rock, because rocks are quite complicated,” Bickle said. “And laboratory experimental studies can only be done over a short period, when we need to look at a time period of 10,000 years [or more]. It’s like all things looking at natural systems — it’s very difficult to recreate a model for them because they’re so complicated.”
While Bickle notes that this is the first field study to examine the processes taking place in a natural carbon dioxide reservoir in such detail, the results do build on previous research suggesting that such reservoirs are safe when they exist near human communities. A 2011 study, for instance, found that natural carbon dioxide seeps in Italy posed a relatively low risk of death to humans living nearby — there was only a one in 32 million chance of dying from contamination, which the authors pointed out was “significantly lower than that of many socially accepted risks.”
“Public acceptance is always a hurdle to wide-scale deployment,” said Howard Herzog, a senior research engineer and carbon capture expert at the Massachusetts Institute of Technology (who was not involved in the new study), by email. “The more facts we have to assure the regulators and public that carbon dioxide storage is safe and effective, the better. These facts come through studies like this, as well as actual operating experience.”
But Herzog cautioned that the costs associated with carbon capture and storage are likely to remain the biggest challenge to its widespread adoption, since “it is always cheaper to emit carbon dioxide into atmosphere than capture and store.”
He pointed out that more stringent policies forcing the reduction of carbon emissions could encourage the more widespread adoption of the technology. But some environmental organizations still have problems with the practice.
When carbon is captured by an industrial facility, it’s not always just sequestered underground and left alone. Often, it’s sold for use in a process known as “enhanced oil recovery,” which injects the carbon dioxide back into depleted oil reservoirs to help push more oil out of the ground, and thus, draw an additional financial benefit from carbon storage. In this way, some groups have argued that carbon capture helps perpetuate the use of fossil fuels in a world that should be moving away from oil and natural gas entirely.
So addressing the safety concerns associated with carbon storage is one step toward the practice of carbon capture becoming more widely accepted, but certainly not the only — or even the biggest — hurdle. That said, Herzog adds that the new study makes an “important contribution” to the field. And when considering the risks thought to be associated with carbon storage, it’s important to keep things in perspective, anyway, Bickle noted.
“The chances of things going wrong when putting it underground are thousands and thousands of times less than what we know will happen if we just pump it into the atmosphere,” he said.